Refractories are those materials which can withstand very high temperatures. Although there is no well defined dividing line between refractories and non-refractories, most generally recognized refractories have softening temperatures in excess of about 1500.degree. C. The usefulness of refractories depends upon an ability to maintain the mechanical functions at high temperatures, quite often in contact with corrosive liquids and gases. Frequently, they are employed in line furnaces and high temperature vessels. Refractories are also provided in a variety of physical forms and shapes and can be comprised of plastics, ramming mixes, gunning mixes, casting mixes, and the like. In particular, refractory nitrides are useful as crucibles for the melting of metals, and also as components of super-hard cutting tools.
In one method of refractory material synthesis, nitrides are prepared by reacting a metal with nitrogen gas. This method, however, requires high furnace temperatures for extended periods of time.
The strong exothermic heat effects of chemical reactions has been employed as a process for synthesizing nitrogen refractory materials. This combustion process, known as the self-propagating high temperature synthesis (SHS), has been utilized by numerous investigators.
Merzhanov, et al, has disclosed a process for the synthesis of refractory inorganic compounds such as carbides, nitrides, borides, sulfides, and silicides. Refractory inorganic compounds are formed utilizing the SHS process with the direct interaction of two chemical elements, one of which, the fuel (usually a metal), is in the condensed state. The other, the oxidizing agent (non-metal), is either in the condensed or in a gaseous state. The combustion process is carried out in either a constant pressure vessel or in special reactors, and initiated with an igniting device.
Borovinskaya, et al, disclosed a similar approach employing an SHS process with the production of various refractory inorganic compounds. This process is directed to the synthesis of titanium nitrides, and employs a high nitrogen pressure in the range of about 500-4500 atm. Nitrogen and titanium of various compositions which range in stoichiometry from TiN.sub.0.5 to TiN.sub.0.99 are obtained. In this case, high pressure equipment is required to reach full conversion.
One serious drawback of the SHS process is the low percent conversion of metal to nitride. The high adiabatic temperatures of the process, e.g., 3000.degree.-4800.degree. C., cause the metal to melt as the combustion fron propagates through the material. The molten metal forms an effective barrier to the inward diffusion of the nitrogen gas from outside the material.
Complete conversion is more likely to occur when high pressure N.sub.2 gas is employed, e.g., 100-5000 atm. Unfortunately the high pressure equipment complicates the process and significantly reduces the advantage over conventional production methods.
It would be an advancement in the art of refractory inorganic material synthesis to provide a method for synthesizing such materials without high pressures. It would also be desirable to provide a method of synthesizing inorganic refractory materials resulting in nearly complete combustion of the starting material, and which process is energy efficient. Of particular importance would be to achieve these goals in forming refractory metal nitride from oxides of the transition metals from the groups III-A, IV-A, III-B, IV-B groups or a rare earth metal oxide.